With the rapid development of network technologies, IM has become a common way of communications over networks. When users use an IM tool to chat, a server would automatically save messages transmitted and received by an IM chat window to form chat records, which are saved on a client device in the form of files. IM tools typically provide a function for searching historical messages. When a user needs to review a historical message, he/she just needs to retrieve a corresponding file of chat records, type in a corresponding keyword, and then can acquire the desired historical message.
In the prior art, however, only plain text information transmitted or received by users can be searched. During chatting, a user can type in or receive non-text information, such as URL (Uniform Resource Locator), sharing of account information, photos, or audio or video files. It is impossible for the user to search this type of non-text information.